1. Field of the Invention
This invention relates to smectic liquid crystals, and more precisely the object of the invention are novel compounds with anticlinic smectic phase and their mixtures exhibiting orthoconic antiferroelectric liquid crystalline phase (OAFLC), showing high chemical stability and a large helical pitch value. Such mixtures are applied as liquid crystalline medium in information display devices, optical valves, holography and especially, considering high contrast and wide viewing angle, they have favorable properties for digital projectors and flat graphical screens.
2. Background of the Invention
Smectic antiferroelectric liquid crystals (AFLC) enable the use of a cheaper passive matrix technology with the same image quality which can be obtained by using active matrices.
The flaw of most known AFLC materials is quite low dynamic contrast, spoiled by so called pretransitional effect, as shown in FIG. 1. This disadvantage may be removed if the orthoconic liquid crystalline medium is used as described in U.S. Pat. No. 6,919,950 and this liquid crystal medium exhibits a long helical pitch. FIG. 1 schematically presents the voltage dependent light transmission T characteristics for a flat-parallel cell placed in a birefractive system, filled with a) normal (low angle) and b) orthoconic antiferroelectric smectic liquid crystal. The fault is induced in thin cell filled with a liquid crystal with short helical pitch (smaller than cell gap) because the helicoidal structure is not completely unwound. This flaw is removed and high contrast of imaging is available, if the direction of the director (a unit vector along the average direction of the spatial orientation of the main molecular axes of inertia) in the smectic layers in the antiferroelectric phase is tilted at an angle close to optimal 45° to the smectic layer normal. Such antiferroelectric material placed in a thin cell is ordered due to surface action in such a way that in a whole cell a homogeneous direction of layer normal is set, and the helix disappears completely. This is possible when the helical pitch value measured in the free space exceeds the gap of cell filled with antiferroelectric liquid crystal.
In a single smectic layer the director is tilted at the angle of 45° to layer normal and in neighboring layers the directions of the director create angle of 90°. The same angle is observed for the directions of optical axes of the orthoconic antiferroelectric smectic during switching between opposite E(+) and E(−) states. This gives the name of the concerned material: Surface Stabilized Orthoconic Antiferroelectric Liquid Crystal (SSOAFLC).
Homogeneously ordered thin slab of the SSOAFLC without influence of any external fields, is a uniaxial medium, with a negative value of the optical anisotropy and with the optical axis perpendicular to SSOAFLC slab in a cell. For a light beam incident perpendicularly to this slab, SSOAFLC behaves like an optically isotropic medium. Additionally when the ε1≈(ε2+ε3)/2 is met (where ε1, ε2, ε3 are main components of the tensor of the dielectric permittivity) it becomes an optically isotropic medium for all directions of the light incidence [J. Appl. Phys., 93(8), 4930-4932 (2003)].
The optical properties of a thin layer of the SSOAFLC at the chiral anticlinic, antiferroelectric state, make defects of the local disorientation of smectic layers (hence the optical axis) not visible. In such conditions, for cells with SSOAFLC in a birefractive system, a perfect dark state is generated [Adv. Funct. Mater., 11, 87-94 (2001)].
After external electric field is applied to a thin slab of SSOALFC in the direction of smectic layers, it switches from the anticlinic (antiferroelectric) structure to the synclinic (ferroelectric) one, which for a light beam at incidence normal to the SSOAFLC slab is a optically positive, optically biaxial medium (with the orientation of the optical axis in the cell plane and perpendicular to the direction of applied electric field). Considering perfect dark state generated by orthoconic antiferroelectric structure, a large optical contrast on transition from dark state to bright state is observed. Advantages and possibilities of applying of this structure for display fabrication have been described at first time in U.S. Pat. No. 6,919,950.
First known compounds enabling the formulation of multicomponent orthoconic mixtures were esters with perfluoroalkanoyloxy group in the terminal chain. They were described in our patent PL 186152 and later papers [Polish J. Chem., 76, 273-284 (2002); Ferroelectrics, 309, 83-93 (2004)].
Using these compounds antiferroelectric mixture W-107 [Ferroelectrics, 244, 115-128 (2000)] was obtained and lately many other. Among them the W-193B mixture is best known [Displays, 25, 9-19 (2004)].
At the room temperature all of those mixtures exhibit short helical pitch p (where the helical pitch p is smaller than 1 μm) which rises upon the increase of the temperature (see FIG. 2, curve a).
In FIG. 2 temperature dependencies of the wave length of the selectively reflected light have been compared for the different antiferroelectric liquid crystal mixtures a) W-193B and b) a mixture obtained according to the invention. Vertical axis is for values of the wavelength λmax at which maximum of the selective light reflection appears. The horizontal line is for the reduced temperature T-TcA, (where TcA is maximum temperature of antiferroelectric phase existence).
Spectrophotometric method is most easily used to measure the value of the helical pitch length p at the range of 0.2-2.0 μm utilizing the phenomenon of the selective light reflection on periodic helicoidal smectic liquid crystal structure. For circularly polarized light beam of wave length λmax, at the normal incidence to the (O)AFLC slab, the condition p=λmax/n (where n is average refractive index of smectic antiferroelectric liquid crystal) is satisfied hence the transmitted light experiences the drop of the intensity by about 50%. It occurs while one portion of circularly polarized light experiences the total internal reflection on periodic helicoidal structure of smectic liquid crystal. When the absorption maximum is observed for the wavelength of λmax, the value of the helical pitch p can be calculated using formula p=λmax/n. The helical pitch longer than 2 μm can be measured by i.e. using wedge cell method or can be evaluated by observation of so called dechiralisation lines in the flat parallel cell.
Components and mixtures mentioned above are characterized by the small values of the helical pitch p, growing with the temperature increase. Helical pitch p is very small at the near room temperature range and rises at the high temperatures (which are rather rarely used). Application of such materials demands very thin cells of thickness below 1 μm. Manufacturing of such a thin cell is technologically complicated; furthermore in such a cell the ferroelectric (synclinic) ordering, instead of the antiferroelectric (anticlinic) one is often observed [Ferroelectrics, 344, 177-188 (2006)]. Moreover, in submicrometer cells, the surface action operates deeply inside the volume of SS(O)AFLC, what has a negative influence on the relaxation process to the antiferroelectric state after disappearance of the driving electric field.
Achiral orthoconic anticlinic compounds have not yet been discovered. Hopefully, they could be used for obtaining of an OAFLC mixture with any helical pitch value. This seems to be possible by doping of an achiral AFLC with the chiral compound of a high twisting power. The same method is successfully used for the preparation of ferroelectric mixtures.
Also orthoconic smectic antiferroelectrics as individual compounds as well as their mixtures with long pitch are no yet known. They could be obtained, either from chiral compounds with large helical pitch, or from achiral anticlinic smectics C or racemates created from pairs of opposite enantiomers of known antiferroelectrics by introducing some amounts of one enantiomer to obtain enantiomer excess.
Unfortunately, decreasing of optical purity of an antiferroelectric compound or its complete racemization causes very strong destabilization or even complete disappearance of anticlinic (antiferroelectric) phase respectively [Ferroelectrics, 309, 83-93 (2004)]. For example, optically active ester of formula A (existing in the form of enantiomer R or S)
is an orthoconic antiferroelectric compound and its phase transitions are:    Cr 47.6 SmCA*91.1 SmC*110.6 SmA 118.5 Isobut its racemic mixture (equimolar mixture of R and S enantiomers) has following phase transitions:    Cr 55.8 SmC 118.4 SmA 121.1 Iso
Therefore until now it was not possible to expand the helical pitch of the antiferroelectric mixture by decreasing optical purity. The similar change of the phase sequence has been observed for compounds with different length of oligomethylene spacers in this family of benzoates as well as for similar biphenylates and many other compounds.